<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      function isRunYear(year) {
        if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) {
          return true
        } else {
          return false
        }
      }
      function day() {
        if (isRunYear(prompt())) {
          alert('二月有29天')
        } else {
          alert('二月有28天')
        }
      }
      day()
    </script>
  </body>
</html>
